Disc wood chippers typically contain a shaft 14 for rotating a disc about a horizontal axis. The disc typically includes a plurality of radially disposed slots. Primary chipping knives are mounted at the entrance to each slot which extend radially along the length of the opening. The blade of each primary knife protrudes outwardly from the front face of the disc in order to slice chips, also referred to as wood chip slices, from a wooden work piece that is brought into contact with the front face of the disc while it is rotating. The wood chip slices leaving the primary knives generally have a width that can equal the diameter of the material as measured in the plane of the disc at the chipper's feed opening and a length that is determined by various machine parameters. The width of the wood chip slices can be greater than the length of the wood chip slices leaving the primary knife. As a result, these wood chip slices often do not lend themselves for use in downstream processing units without further reduction of the size of the individual wood chip slices. Most machines for further reducing the size of the wood chips consume a substantial amount of energy.
Counter knives have been mounted in the slots of some chippers to engage the chip slices leaving the primary knives in order to further reduce the widths of the slices. These counter knife devices are generally not very efficient and fail to utilize a substantial portion of the energy that is carried by the chip slices leaving the primary chipping knife.